Antibody levels wane over time, but this primarily impacts protection against infection. Protection against severe disease, hospitalization, and death is reduced only slightly over time by comparison.
Boosters restore and often exceed the original peak protection against infection, and this protection may be more durable than that after only 2 doses.
Title: Vaccine effectiveness and duration of protection of Comirnaty, Vaxzevria and Spikevax against mild and severe COVID-19 in the UK (2021-10-06)
"We observed limited waning in vaccine effectiveness against hospitalisation and death more than 20 weeks post-vaccination with Vaxzevria or Comirnaty. Waning was greater in older adults and those in a clinical risk group, suggesting that these individuals should be prioritised for booster doses."